It is previously know to use industrial robots for carrying out automatic welding along a weld joint and to use in that connection a sensor mounted adjacent to the welding tool supported by the robot hand to cause the welding tool to trace along the weld joint. The welding can therefore be carried out automatically in a satisfactory manner, to a smaller or greater extent independently of variations between the weld pieces in terms of the extension of the weld joint.
Equipment of this kind is previously known, for example from U.S. patent application Ser. No. 597,298 (filed on Apr. 6, 1984 in the names of Edling et al) and from U.S. Pat. Nos. 4,417,127 and 4,501,950.
Equipment of the above-mentioned kind can be employed also for other types of work operations than welding, such as, e.g. for spreading glue or laying out strings of a sealing compound.
In equipment of the above-mentioned kind, the robot can be programmed to track a pre-programmed path, the sensor giving the necessary corrections to the pre-programmed path. According to another alternative, the sensor and the control system of the robot can be designed in such a way that no pre-programming of a path is necessary at all. In connection with welding, for example, the sensor moves along the weld joint a certain distance ahead of the welding tool and continuously determines the position of the weld joint. The welding tool is controlled to track the path determined by the sensor. In the latter alternative, the necessary programming work is greatly reduced. In principle, the robot need only be positioned with the welding tool at the initial point of the weld joint and in such a way that the weld joint is located within the measuring range of the sensor, whereafter the welding procedure can be started.
It should be noted that the path which is tracked by the robot hand (the tool) is not necessarily a defined path, but the expression "path" relates to the curve that describes the course of the robot hand during the required operation. It may have any arbitrary shape and orientation, and in each particular case it is determined by the work object via the sensor.
In equipment of the kind described above, it has so far been impossible to obtain, in a simple manner, an interruption of the required operation at precisely the desired point on the workpiece. Owing to unavoidable variations as to position, orientation and dimensions between the individual workpieces the position of the desired end point may vary considerably relative to the object. Hitherto, therefore, it has normally been necessary, at each individual workpiece and prior to the work-operation, to first manually move the robot to the end point of the working path and store the coordinates of this point. Thereafter, the robot is manually moved to the initial point of the working path, whereupon the working procedure is started. The robot has thus become programmed in such a way that it stop and interrupts the work operation when a certain predetermined distance has been covered. This method is time-consuming and requires a relatively extensive operator effort.
The invention aims to provide industrial equipment of the kind mentioned in the introduction, in which the operator's work is reduced to a minimum by eliminating the need to program the end point of the work operation for each individual workpiece.
From U.S. Pat. Nos. 3,860,862 and 4,220,903 equipment is known in which photoelectric detectors are provided for automatic tracking of a line on, for example, a drawing. On the basis of signals from the detector, a work tool, for example a gas cutting torch, is controlled along a desired path on a workpiece. By providing the drawn line to be tracked with cross strokes, interruptions, or portions with a deviating line width, it is possible to initiate, for example, the start/stop of a work operation, or a change of the velocity of movement by way of the detector. These two publications do not deal with the above-mentioned special problems which arise in connection with an industrial robot which, with the aid of a detector, mounted together with an operating member on the robot hand is to track a path determined by the work object itself, for example a seam to be welded.
U.S. Pat. No. 4,623,778 describes an automatic welding machine in which a photoelectric detector is arranged to cause the welding tool to track a seam of a workpiece. If the detector loses the seam, the operation is interrupted after a predetermined time. However, no method is suggested for causing an operation carried out by an industrial robot to be terminated at a desired point on a workpiece, independently of variations in position, orientation and dimensions between individual workpieces.